The present invention relates generally to the field of applying diffusion coatings to industrial parts, and in particular to a new and useful process for chromizing commercial quantities of small, individual, ferrous-based industrial parts in a rapid and cost efficient manner.
Chromizing is a process of producing a chromium diffusion coating on ferrous-base components to improve corrosion resistance, especially at elevated temperatures. Chromizing was developed to produce an integral protective surface coating on components exposed to extreme conditions to enhance their usable life.
Chromizing of parts such as bolts, screws, studs and the like is commonly accomplished using a powder pack cementation technique. In this technique, the parts are packed into dry powder mixtures of aluminum oxide, chromium, and activator salts within a retort, which is then sealed and heated. During the heating, the temperature of the retort is raised to an elevated level and held for a predetermined amount of time. A chemical reaction takes place during the heating process which causes a surface layer of high chromium content to be diffused into the iron of the ferrous-based parts. The retort and parts are cooled and the parts, now with a layer of chromium coating are removed from the retort.
However, the pack cementation method has significant drawbacks when used to coat such small, individual parts. The coating thicknesses often vary widely between individual coated parts, or across the surfaces of larger parts. This is caused by the poor thermal conductivity of the powder pack and the resulting very slow and non-uniform heating rates for the packed parts. Another drawback of this process is that if the parts come into direct contact with each other, the chromium coating diffusion bonds the parts together at these contact points. Further, a large volume of powder waste is generated by the process.
U.S. Pat. No. 4,904,501 to Davis, assigned to The Babcock & Wilcox Company, discloses an improved method for chromizing the surface of a ferritic boiler component which involves applying aqueous coating compositions containing chromium directly to the surface to be chromized. The aqueous compositions include chromium, alumina, a binder of ammonium alginate or methyl cellulose, and a halide activator.
Other processes which improve on the pack cementation technique are disclosed in U.S. Pat. Nos. 5,135,777 and 5,041,309 to Davis et al., also assigned to The Babcock & Wilcox Company. In these processes, an inert refractory container or a ceramic carrier is first coated with a slurry of a diffusion composition. The diffusion composition may contain chromium as the diffusion element. Then the carrier or container is inserted within or placed upon a workpiece to be coated, and heat treated to diffuse a coating onto the workpiece. The carrier or container is removed following the heat treatment. This process is sometimes referred to as a "blanket" process, since the inert carrier is often a spun alumina-silica fiber paper or blanket.
The blanket chromizing process is useful for obtaining uniform coatings with minimal waste on large surfaces or large continuous areas. However, that process has limited benefits when chromizing small individual parts, such as threaded bolts, studs and the like. For example, to chromize carbon steel studs (such as those weld-attached to boiler tubes), special handling is required when using pack cementation and blanket processes to ensure proper positioning to avoid fusing the studs together at points of metal-to-metal contact during the heat treatment process. This special handling is a time-consuming process that has limited benefit, since the parts often shift during the process and thus still become fused together. Additional processing time is then required to separate the usable parts and reject unevenly coated pieces.